1. Field of the Invention
The present invention relates to a piezoelectric element having electromechanical conversion functionality, a liquid jetting head featuring this piezoelectric element, and a method for manufacturing these. More particularly, the invention relates to a piezoelectric element that has excellent piezoelectric characteristics when used in a liquid jetting head, to a liquid jetting head featuring this, and to a method for manufacturing these.
2. Related Art
Inkjet recording heads and other liquid jetting heads use piezoelectric elements as drive sources for expelling ink drops and other liquid drops. Such piezoelectric elements commonly comprise piezoelectric thin films and top and bottom electrodes disposed on both sides thereof.
Piezoelectric elements with improved characteristics have been developed by designing a thin-film crystal structure that comprises lead zirconate titanate (PZT), and forming a Ti nucleus on the bottom electrode. For example, a PZT thin film that has a rhombohedral crystal structure and a specific degree of orientation is disclosed in Japanese Patent Publication No. H10-81016. Further, a piezoelectric element in which a Ti nucleus is formed on an Ir bottom electrode is disclosed in Japanese Patent Publication No. H8-335676.
In conventional steps for manufacturing piezoelectric elements, however, problems are encountered such that the desired degree of orientation of crystal planes in a piezoelectric thin film is difficult to obtain in a stable manner. With such piezoelectric elements, high piezoelectric characteristics are difficult to obtain in a stable manner as a result of the fact that the degree of orientation of the crystal plane is unstable. This is a factor that makes it difficult to achieve an adequate printing performance in an inkjet recording head or printer.
Meanwhile, forming a PZT thin film on a platinum film by a sol-gel technique in six separate cycles is disclosed in Japanese Patent publication No. H6-5948 as a method for producing memory cells, capacitors, and other ferroelectric thin-film elements. In particular, the annealing temperature in the first two steps is set higher than in the other four steps, and the annealing time in the last two steps is made greater than in the other four steps.
However, even this method fails to yield an orientation that would be suitable for piezoelectric elements.
Another drawback is that the reliability is not sufficient because significant in-film stress develops in the conventional piezoelectric material films and bottom electrodes.
In addition, no consideration is given in the conventional steps for manufacturing piezoelectric elements to the take-up of the Ti nucleus by piezoelectric thin-film. For this reason, a problem is encountered whereby areas that are rich in Ti and have a discontinuous composition remain on the interface between the piezoelectric thin film (PZT) and the bottom electrode (BE), as shown in FIGS. 8A and 9A. Consequently, it is sometimes difficult to ensure adequate piezoelectric characteristics or reliability as a result of the fact that the Zr/Ti ratio of a piezoelectric thin-film is nonuniform in the film thickness direction and that in-film stress remains in the piezoelectric thin-film.